CODEL TunnelTech 100 Series, Техническое руководство

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Technical Manual
Page 3
CODEL
Issue : A Rev. : Date : 29/06/2015 Ref. : 100190
1.7. Auto Zero
The measurement of opacity is dependent upon the optical surfaces of the instrument remaining clean. If the
surfaces of the instrument lenses become contaminated it will reduce the intensity of the received light and
increase the opacity measured value. Over a period of time, it is normal to observe a slow build-up of optical
contamination resulting in a steady increase in the measured opacity value. This appears as a persistent
positive output drift.
There are two cures for this problem. One is to clean the optical surfaces on a regular basis. However, in a road
tunnel regular access is not usually possible without a tunnel closure.
The second method is to automatically compensate for such a build-up of contamination. The technique used by
TunnelTech 101
relies on the assumption that there will always be periods of low real opacity during the course
of the day. These periods usually occur at night when traffic loading is low. These periods provide a reference
condition for the measurement.
During normal operation, the effect of contamination is to slowly increase the measured value of opacity. To
compensate for this the instrument is programmed to slowly reduce its measured value of opacity at a rate
faster than that at which contamination would increase.
Over a period of time, this will result in the instrument reading slightly low. However, during the periods of zero
opacity, this effect would cause the instrument to be displaying a negative opacity. Since negative opacity is an
impossible value the negative offset can be quickly rectified and the calibration corrected.
In this way periods of zero opacity, whenever they occur, are utilised to correct the calibration of the
measurement allowing long periods of operation without maintenance and the cleaning of optical lenses.
1.8. Temperature Measurement
The instrument is provided with a PT100 temperature sensor mounted on the transmitter unit, to enable a
continuous measurement of air temperature